This invention relates to a control system for a hydraulically driven vehicle.
A hydraulically driven vehicle is generally provided with a pair of independent closed loops, each for driving one side of tracks or wheels. Each closed loop includes a variable displacement hydraulic pump driven by a prime mover such as an engine and a variable displacement hydraulic motor driven by the variable displacement hydraulic pump.
Displacement volume of each variable displacement hydraulic pump is controlled by changing the angle of a swash plate, which is connected to a servo control and operated thereby.
Each servo control comprises a servo cylinder operatively connected to the swash plate of each variable displacement hydraulic pump and a directional control valve connected with the servo cylinder. Each directional control valve is provided with a restrictor at the input side thereof and connected to a fixed displacement pump through the restrictor. The directional control valves are the pilot-operated type and pilot ports of which are connected with a pair of manually operated steering valves, which in turn are connected with a manually operated directional control valve for speed control and forward and reverse change-over of the vehicle. This manually operated directional control valve is connected with another fixed displacement charge pump. Since each directional control valve of the servo control is provided with the restrictor at the input side thereof, when the manually operated directional control valve for speed control and forward and reverse change-over is operated, hydraulic fluid flow introduced into the servo cylinders through the directional control valves is restricted, thereby preventing a sudden change in angle of the swash plates. As a result, shocks occuring at the acceleration and decelaration of the vehicle can effectively be reduced.
However, due to provision of the restrictors at the input sides of the directional control valves of the servo controls, even when one of the manually operated steering valves is operated, hydraulic fluid flowing into one of the servo cylinders is restricted, thereby causing a response lag of the changing in angle of the swash plate which may result in an excessive turning of the vehicle against the intention of the operator.